Gate electrodes made of highly conductive materials such as metals, silicides or highly doped poly-silicon, positioned between the source and drain electrode, are used to control the two-dimensional charge carrier gas channel in wide bandgap semiconductor devices based on GaN or GaAs layers. The gate electrodes typically form a Schottky diode on top of the two-dimensional charge carrier gas channel, or are separated from the two-dimensional charge carrier gas channel by an insulating material such as SiO2 and/or a thin GaN or AlGaN layer. By applying a negative voltage to the gate, the underlying two-dimensional charge carrier gas channel is disrupted and as a consequence the transistor is in a blocking state.
With GaN HEMTs (high electron mobility transistors), the net charge of the gate or field electrode only depends on the capacity of the electrode with respect to its surrounding area and the applied voltage. In case a blocking voltage is applied to the drain electrode, almost the complete mirror charge in the form of free electrons is provided on the field electrode. The mirror charge counterbalances the opposite charge that builds up within the drain in the transistor blocking state, so as to bring about electrical equilibrium within the device. If there is no field electrode present, the complete mirror charge is provided on the gate electrode. Owing to the highly mobile nature of free electrons in metals, the mirror charge is almost completely positioned on the side of the electrode (field or gate electrode) facing the drain electrode. The mirror charges concentrate almost to a point at the drain edge of the electrode, causing very high electric field peaks particularly towards the drain edge of the electrode. These high electric field peaks cause the injection of charge carriers into neighboring insulating layers and interfaces between layers of different materials, respectively, thereby changing the static and dynamic electrical behavior of the transistor. Such trapping effects are detrimental to device operation.